Agent Payments with Visa TAP

Visa Trusted Agent Protocol (TAP) brings cryptographic identity verification to agentic commerce. Using RFC 9421 HTTP Message Signatures, agents prove their identity by signing HTTP requests with their private keys. TAP enables transparent, verifiable agent payments without relying on API keys, tokens, or session state—just cryptographic proof of identity at the edge.

Prerequisites

What You'll Learn

What is Visa TAP?

Visa Trusted Agent Protocol (TAP) is a payment protocol designed for the agentic economy. Unlike traditional payment methods that rely on API keys or bearer tokens, TAP uses RFC 9421 HTTP Message Signatures to cryptographically bind payments to agent identities.

Step 1: Create a Visa TAP Payment Challenge

Use the create_payment_challenge MCP tool to generate a Visa TAP payment challenge for a resource:

curl -s -X POST "https://mcp.tenzro.network/mcp" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: $(uuidgen)" \
  -d '{
    "jsonrpc": "2.0",
    "id": 7,
    "method": "tools/call",
    "params": {
      "name": "create_payment_challenge",
      "arguments": {
        "protocol": "visa-tap",
        "resource": "/api/inference",
        "amount": 250000,
        "asset": "TNZO",
        "recipient": "0x0000000000000000000000000000000000000001"
      }
    }
  }'

Response:

{
  "jsonrpc": "2.0",
  "id": 7,
  "result": {
    "content": [
      {
        "type": "text",
        "text": {
          "challenge_id": "visa_tap_ch_x9y8z7w6v5u4",
          "protocol": "visa-tap",
          "resource": "/api/inference",
          "amount": 250000,
          "asset": "TNZO",
          "recipient": "0x0000000000000000000000000000000000000001",
          "chain_id": 1337,
          "required_headers": [
            "@method",
            "@authority",
            "@path",
            "content-digest",
            "content-type"
          ],
          "max_signature_age_secs": 480,
          "domain_binding": "api.tenzro.network",
          "created_at": "2026-03-24T14:20:00Z",
          "expires_at": "2026-03-24T15:20:00Z"
        }
      }
    ],
    "isError": false
  }
}

The challenge includes required_headers (the HTTP fields that must be signed), max_signature_age_secs (480 seconds = 8 minutes), and domain_binding to prevent cross-domain signature reuse.

Step 2: Sign the Request with RFC 9421

The agent must sign the HTTP request using RFC 9421 HTTP Message Signatures. The signature covers specific HTTP fields (method, authority, path, content-digest, content-type) and includes metadata (created timestamp, keyid, nonce).

Rust example using Tenzro SDK:

use tenzro_payments::visa_tap::VisaTapClient;
use tenzro_crypto::signers::Ed25519Signer;

// Agent's signing key
let signer = Ed25519Signer::from_private_key(&agent_private_key)?;
let agent_did = "did:tenzro:machine:agent123";

// Create TAP client
let client = VisaTapClient::new(signer, agent_did);

// Build the HTTP request parts
let request_parts = HttpRequestParts {
    method: "POST",
    authority: "api.tenzro.network",
    path: "/api/inference",
    headers: vec![
        ("content-type", "application/json"),
        ("content-digest", "sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:"),
    ],
    body: r#"{"model":"gemma4-9b","prompt":"What is Tenzro?"}"#,
};

// Sign the request (produces Signature-Input and Signature headers)
let signed_request = client.sign_request(&request_parts).await?;

println!("Signature-Input: {}", signed_request.signature_input);
println!("Signature: {}", signed_request.signature);

The resulting HTTP request headers:

POST /api/inference HTTP/1.1
Host: api.tenzro.network
Content-Type: application/json
Content-Digest: sha-256=:X48E9qOokqqrvdts8nOJRJN3OWDUoyWxBf7kbu9DBPE=:
Signature-Input: sig1=("@method" "@authority" "@path" "content-digest" "content-type");created=1711289600;keyid="did:tenzro:machine:agent123";nonce="abc123xyz456"
Signature: sig1=:MEUCIQDXlI2fZH+sTmSu6Z1xN3vKjP5wQmN7rL8sF4tG9yH3xQIgRpLsK2mN8vT6wY1zU4cD5eF7gH9iJ0kL2mN4oP6qR8s=:

{"model":"gemma4-9b","prompt":"What is Tenzro?"}

Step 3: Verify Payment Credential

Use the verify_payment MCP tool to verify the signed request and settle the payment on-chain:

curl -s -X POST "https://mcp.tenzro.network/mcp" \
  -H "Content-Type: application/json" \
  -H "Accept: application/json, text/event-stream" \
  -H "Mcp-Session-Id: $(uuidgen)" \
  -d '{
    "jsonrpc": "2.0",
    "id": 8,
    "method": "tools/call",
    "params": {
      "name": "verify_payment",
      "arguments": {
        "challenge_id": "visa_tap_ch_x9y8z7w6v5u4",
        "protocol": "visa-tap",
        "payer_did": "did:tenzro:machine:agent123",
        "amount": 250000,
        "asset": "TNZO",
        "signature": "MEUCIQDXlI2fZH+sTmSu6Z1xN3vKjP5wQmN7rL8sF4tG9yH3xQIgRpLsK2mN8vT6wY1zU4cD5eF7gH9iJ0kL2mN4oP6qR8s=",
        "signature_input": "sig1=(\"@method\" \"@authority\" \"@path\" \"content-digest\" \"content-type\");created=1711289600;keyid=\"did:tenzro:machine:agent123\";nonce=\"abc123xyz456\""
      }
    }
  }'

Response:

{
  "jsonrpc": "2.0",
  "id": 8,
  "result": {
    "content": [
      {
        "type": "text",
        "text": {
          "verified": true,
          "receipt": {
            "payment_id": "visa_tap_pay_u4v5w6x7y8z9",
            "challenge_id": "visa_tap_ch_x9y8z7w6v5u4",
            "payer_did": "did:tenzro:machine:agent123",
            "amount": 250000,
            "asset": "TNZO",
            "recipient": "0x0000000000000000000000000000000000000001",
            "settlement_tx": "0x1122334455667788...",
            "settled_at": "2026-03-24T14:22:35Z",
            "stages_passed": [
              "HeaderExtraction",
              "KeyRetrieval",
              "TimestampValidation",
              "ReplayPrevention",
              "DomainBinding",
              "CryptographicVerification",
              "ResultConstruction"
            ]
          }
        }
      }
    ],
    "isError": false
  }
}

The response shows verified: true and a receipt with the settlement transaction hash. The stages_passed array documents which verification stages succeeded.

Step 4: Understand the 7-Stage Pipeline

Visa TAP verification runs through a rigorous 7-stage pipeline to ensure cryptographic integrity and prevent fraud:

// 7-Stage Visa TAP Verification Pipeline

// Stage 1: Header Extraction
// Parses Signature-Input and Signature headers
// Example: sig1=("@method" "@authority" "@path" ...);created=1711289600;keyid="did:tenzro:machine:agent123"

// Stage 2: Key Retrieval
// Resolves agent DID via TDIP registry
// Fetches Ed25519 public key from identity document

// Stage 3: Timestamp Validation
// Checks 'created' parameter within max_signature_age_secs (480 seconds = 8 minutes)
// Prevents stale signatures from being replayed

// Stage 4: Replay Prevention
// Checks nonce against cache (in-memory or Redis)
// Each nonce can only be used once per challenge

// Stage 5: Domain Binding
// Verifies @authority matches required domain ("api.tenzro.network")
// Prevents cross-domain signature reuse

// Stage 6: Cryptographic Verification
// Constructs canonical signing payload per RFC 9421
// Verifies Ed25519 signature with agent's public key

// Stage 7: Result Construction
// Builds VerificationResult with all metadata
// Returns receipt with settlement transaction hash

Step 5: Check Settlement Receipt

The settlement receipt provides a complete audit trail of the payment:

{
  "payment_id": "visa_tap_pay_u4v5w6x7y8z9",
  "challenge_id": "visa_tap_ch_x9y8z7w6v5u4",
  "payer_did": "did:tenzro:machine:agent123",
  "amount": 250000,
  "asset": "TNZO",
  "recipient": "0x0000000000000000000000000000000000000001",
  "settlement_tx": "0x1122334455667788...",
  "settled_at": "2026-03-24T14:22:35Z",
  "stages_passed": [
    "HeaderExtraction",
    "KeyRetrieval",
    "TimestampValidation",
    "ReplayPrevention",
    "DomainBinding",
    "CryptographicVerification",
    "ResultConstruction"
  ]
}

How It Differs from MPP

Security Considerations

• Private key protection: Agent private keys must be stored securely in TEE enclaves or hardware wallets
• Nonce uniqueness: Every request must use a unique nonce to prevent replay attacks
• Timestamp validation: Enforce strict time windows (8 minutes) to limit signature lifetime
• Domain binding enforcement: Always verify @authority matches expected domain
• TDIP delegation scopes: Verify agent delegation scopes allow the payment amount, protocol, and chain
• Content-Digest integrity: Verify SHA-256 digest of request body matches content-digest header

What's Next?

• Payments with Mastercard Agent Pay — Learn Know Your Agent (KYA) verification and agentic token issuance
• Build an AI payment agent — Create an autonomous agent that uses Visa TAP
• RFC 9421: HTTP Message Signatures — Specification for HTTP signature format
• TDIP Documentation — Understand decentralized identity and delegation scopes